What's the point of using Docker on top of a virtual machine?

Question

New to Docker...Docker is described as an alternative to virtual machines. I've read a bit now and that makes sense. What I don't fully understand is why you would use Docker on top of a virtual machine eg. this is exactly what AWS now offer with beanstalk.

Is it simply because most hosting is VM based, and Docker gives you flexibility so even though it's not replacing the VM, it's still (potentially, depending on use-case) worth using?

Answer 1

The use case which works for us is that you can use the same docker image on top of a real box, virtual machine, different operating systems, etc. and have the same controlled environment.

Even if you only plan to run this on one type of machines (e.g. on EC2 virtual machines), you still probably develop it on your laptops or desktops, so it may make sense to use it.

And the good part is, that the Docker overhead is very small (or at least that's the idea), so it wouldn't impact the performance even when run on virtual hardware. This is in contrast with "real" virtual machines (no pun intended, but it sounds good :)), which would bring a large overhead in similar scenario.

Answer 2

Everything at Google runs in a Linux container, according to Infrastructure VP Eric Brewer.

But everything at Google doesn't run in a VM.

Yes, KVM is used when a different OS is needed, and the public Compute Platform service (for now) runs Docker inside VMs, perhaps for stronger isolation and control. Otherwise, most of Google's containers run on bare metal. Why add extra overhead?

If you know differently, please correct me.

In the enterprise today, the ratio of VMs to containers may be 99 to 1. How long until it's 30/70 or 20/80?

While Kubernetes and other container orchestration tools may never reach vSphere's polish and ease of use, I wouldn't want to be a virtualization vendor right now.